Current automotive vehicle manufacturing operations need to be adaptable to make different parts or components with the same equipment and with minimal modification of the equipment or tools. Such operations include, for example, the joining of two sheet metal layers by spot welding. Vehicle body panels such as doors, hoods, deck lids and liftgates are often assembled by joining inner and outer panels stamped from sheet metal of suitable aluminum or ferrous alloys. The thickness of each sheet metal layer may vary from less than one millimeter to more than four millimeters. Electrical resistance spot welding is often used to join such inner and outer panels and other sheet metal parts.
In the case of sheet metal body components, flats or flanges of two or three components are placed together and then a series of spot welds are made to securely attach the panels. Welding practices have been developed for such spot welding operations. Good welding practices are particularly critical in joining aluminum sheet alloys because of the high electrical and thermal conductivity of the material and the omnipresent oxide coating on the surface. The spot welding operation is accomplished by assembling the parts in a suitable fixture and pressing welding electrodes against opposite sides of the touching parts at the intended weld location. The welding electrodes provide both clamping force and current commutation for the weld.
Welding practice has specified different sized welding tips or electrodes depending upon the metal to be welded and the thicknesses of the layers to be joined. The welding electrodes are part of a welding apparatus including a welding gun that would be moved and actuated to press the electrodes against the assembled layers. The apparatus would then deliver a momentary welding current to the electrodes to effect the weld. The clamping force, the welding current (single phase alternating current, 60 Hz, or rectified direct current) and current duration (several cycles of the 60 cycle current) are also specified for the electrodes to be used. Thus, a particular welding center would require several different welding electrodes to accommodate different part thicknesses.
For example, The Aluminum Association, Inc. publishes “Guidelines to Resistance Spot Welding Aluminum Automotive Sheet.” Table 7 of that Guideline identifies ten different sheet metal thickness steps from 0.6 mm to 3.2 mm for the nominal thickness of the thinner aluminum sheet of the pair to be welded. Two sheets to be joined may be of different thicknesses within such range. Further, successive assemblies in a lineup of welding jobs may vary substantially in thickness. Corresponding to these thickness designations are five different electrode tip diameter and shape specifications. These requirements, based on welding experience, have complicated the creation of a flexible welding center for aluminum sheet metal assemblies of typical varying thicknesses because of the need for so many different welding electrodes.
It is an object of this invention to provide a practice for simplifying the tooling requirements of a welding center or station for accommodating aluminum sheet metal parts of varying thicknesses; for example, sheet thicknesses varying over a range from at least about 0.9 to over 4 mm. Another object of this invention is to provide a “universal” welding electrode of shape and size suitable for use in welding such a range of aluminum sheet thicknesses.